This invention relates to a system for printing or writing with droplets of ink. The invention is particularly concerned with a drop writing system in which a writing head connected to an ink supply forms ink droplets and projects them toward a target, and selected ones of the droplets strike the print target while the unselected droplets return to the ink supply. During a drop writing operation of that type the ink is depleted in two ways. First, there is a portion of the ink which leaves the system because it strikes the print target. Secondly, there is a liquid portion of the ink which evaporates on exposure to air. The evaporation occurs at a higher rate with solvent based inks than with aqueous based inks, and the evaporation loss increases as more of the ink is recirculated. In prior art systems, the ink supply is replenished by adding a specially formulated so-called make-up ink. The formulation depends upon the nature of the printing. If in one installation a substantial amount of the ink is recirculated in relation to the amount of ink expended in printing and in another installation much less ink is recirculated for the same given amount of ink expended in printing, then the make-up ink for the first installation is provided with a greater amount of solvent.
It has been difficult to formulate the make-up inks appropriately to meet the different types of applications for droplet writing systems. Furthermore, it has been costly and a burden to inventory many formulations of make-up inks. Finally the prior art approach of replenishing the ink supply by using selectively formulated make-up inks has not been fully satisfactory because if the printing pattern varies so that the make-up ink is not appropriate because it includes more solvent and less solids than required, or vice versa, then the composition of the ink in the ink supply is modified and the effect is that optimum printing results are not obtained. An ink jet system using a make-up ink replenishing arrangement is shown in Dick et al. U.S. Pat. No. 3,930,258, assigned to A. B. Dick Company, assignee of the present application.